This catalog stores information about data types. Scalar types
("base types") are created with CREATE TYPE.
A complex type is also created for each table in the database, to
represent the row structure of the table. It is also possible to create
derived types with CREATE DOMAIN.

Table 3-22. pg_type Columns

Name

Type

References

Description

typname

name

Data type name

typowner

int4

pg_shadow.usesysid

Owner (creator) of the type

typlen

int2

Length of the storage representation of the type, -1 if variable length

typprtlen

int2

unused

typbyval

bool

typbyval determines whether internal
routines pass a value of this type by value or by reference.
Only char, short, and
int equivalent items can be passed by value, so if
the type is not 1, 2, or 4 bytes long,
PostgreSQL does not have
the option of passing by value and so
typbyval had better be false.
Variable-length types are always passed by reference. Note that
typbyval can be false even if the
length would allow pass-by-value; this is currently true for
type float4, for example.

typtype

char

typtype is b for
a base type, c for a complex type (i.e.,
a table's row type), or d for a derived type (i.e.,
a domain). See also typrelid
and typbasetype.

typisdefined

bool

True if the type is defined, false if this is a placeholder
entry for a not-yet-defined type. When typisdefined is false,
nothing except the type name and OID can be relied on.

typdelim

char

Character that separates two values of this type when parsing
array input. Note that the delimiter is associated with the array
element data type, not the array data type.

typrelid

oid

pg_class.oid

If this is a complex type (see
typtype), then this field points to
the pg_class entry that defines the
corresponding table. A table could theoretically be used as a
composite data type, but this is not fully functional.
Zero for non-complex types.

typelem

oid

pg_type.oid

If typelem is not 0 then it
identifies another row in pg_type.
The current type can then be subscripted like an array yielding
values of type typelem. A
"true" array type is variable length
(typlen = -1),
but some fixed-length (typlen > 0) types
also have nonzero typelem, for example
name and oidvector.
If a fixed-length type has a typelem then
its internal representation must be N values of the
typelem data type with no other data.
Variable-length array types have a header defined by the array
subroutines.

typinput

regproc

Input function

typoutput

regproc

Output function

typreceive

regproc

unused

typsend

regproc

unused

typalign

char

typalign is the alignment required
when storing a value of this type. It applies to storage on
disk as well as most representations of the value inside
PostgreSQL.
When multiple values are stored consecutively, such
as in the representation of a complete row on disk, padding is
inserted before a datum of this type so that it begins on the
specified boundary. The alignment reference is the beginning
of the first datum in the sequence.

Possible values are:

'c' = CHAR alignment, i.e., no alignment needed.

's' = SHORT alignment (2 bytes on most machines).

'i' = INT alignment (4 bytes on most machines).

'd' = DOUBLE alignment (8 bytes on many machines, but by no means all).

Note: For types used in system tables, it is critical that the size
and alignment defined in pg_type
agree with the way that the compiler will lay out the field in
a struct representing a table row.

typstorage

char

typstorage tells for variable-length
types (those with typlen = -1) if
the type is prepared for toasting and what the default strategy
for attributes of this type should be.
Possible values are

'p': Value must always be stored plain.

'e': Value can be stored in a "secondary"
relation (if relation has one, see
pg_class.reltoastrelid).

'm': Value can be stored compressed inline.

'x': Value can be stored compressed inline or in "secondary".

Note that 'm' fields can also be moved out to secondary
storage, but only as a last resort ('e' and 'x' fields are
moved first).

typnotnull

bool

typnotnull represents a NOT NULL
constraint on a type. Presently used for domains only.

typbasetype

oid

pg_type.oid

If this is a derived type (see typtype),
then typbasetype identifies
the type that this one is based on. Zero if not a derived type.

typtypmod

int4

typtypmod records type-specific data
supplied at table creation time (for example, the maximum
length of a varchar column). It is passed to
type-specific input and output functions as the third
argument. The value will generally be -1 for types that do not
need typmod. This value is copied to
pg_attribute.atttypmod when
creating a column of a domain type.

typndims

int4

typndims is the number of array dimensions
for a domain that is an array. (The array element type is
typbasetype.) Zero for non-domains and non-array domains.
This value is copied to
pg_attribute.attndims when
creating a column of a domain type.

typdefaultbin

text

If typdefaultbin is not NULL, it is the nodeToString
representation of a default expression for the type. Currently this is
only used for domains.

typdefault

text

typdefault is NULL if the type has no associated
default value. If typdefaultbin is not NULL,
typdefault must contain a human-readable version of the
default expression represented by typdefaultbin. If
typdefaultbin is NULL and typdefault is
not, then typdefault is the external representation of
the type's default value, which may be fed to the type's input
converter to produce a constant.